1) Field of the Invention
The present invention relates to an image processing apparatus and method of obtaining a high quality reproduced image for a character edge portion. More particularly, this invention relates to an image processing apparatus and method of performing a plurality of adaptive processings based on an edge amount, and being a technique suited for copiers, printers, facsimiles, or the like.
2) Description of the Related Art
There has been a conventional art such that the edge amount of an edge portion in an image input through a scanner or the like is detected to control each image processing according to the edge amount. This is intended to provide an image in which a picture pattern portion is kept smooth by putting emphasis on gradation of this portion while a character edge portion is thin and clear by putting emphasis on sharpness of this portion. Accordingly, various processings related to the sharpness of character edge are considered as controlling of each character edge portion according to the edge amount. The processings of a color image processing apparatus shown in FIG. 1 includes a filtering (edge enhancement and smoothing), an ink generation/undercolor removal processing, a gamma conversion processing, and a halftoning. If a monochrome image is to be processed, all processings but the ink generation/undercolor removal processing are performed. Further, an apparatus that controls a plurality of adaptive processings according to one edge amount or a corrected edge amount obtained by being corrected according to the control target processing has been proposed in recent years.
An apparatus described, for example, in Japanese Patent Application Laid-Open No. 2000-206756 performs adaptive processings on an edge enhancement processing and an ink generation/undercolor removal processing. An edge region is detected from a signal not subjected to an edge enhancement processing, a detected edge amount (Laplacian quantity) is converted using look-up tables (LUT) and the resultant quantity is added to a source signal, thereby realizing edge enhancement. The ink generation/undercolor removal processing is performed after the edge enhancement. In the ink generation/undercolor removal processing, the edge region detected from the signal before being subjected to edge enhancement is enlarged and the densities of respective color components in the enlarged edge region are changed. Further, the adaptive processing for the ink generation/undercolor removal processing has functions of generating an edge amount so that the portions that are enlarged outwardly (those on a background) are canceled and only the portions enlarged inwardly (those on characters) are left, and of changing the densities based on this signal. If a high rate ink generation/high rate undercolor removal processing is conducted to the outer edge which corresponds to a color base background portion of a character on a color base similarly to the character portions, a color difference tends to occur between the outer edge and the color base background portion outside of the outer edge, which is not preferable.
In the image processings shown in FIG. 1, there are processings, other than the halftoning, which inevitably or slightly intentionally control the line width of character portions on image data. These are the filtering and the gamma conversion processing. In the edge enhancement processing in the filtering, line width tends to be larger if a spatial filter that emphasizes low frequency components is used, and tends to be smaller if a spatial filter that emphasizes high frequency components is used.
Generally, it is considered that a “thin yet clear” character edge portion is preferable. However, this does not apply to a low contrast character that is low in density. The emphasis on high frequency components is not necessarily good. It is most important for the low contrast character to be visible and to be readable. In other words, it may be preferable to make the low contrast character thick.
Similar line width control can be conducted by gamma conversion. An example of controlling line width by gamma conversion is shown in FIG. 2. Gamma conversion 1 is conducted in a direction in which the line width is increased, and, in contrast, gamma conversion 2 is conducted in a direction in which the line width is decreased.
As can be seen, although the line width may possibly be changed before and after the filtering or the gamma conversion, the apparatus described in the publication controls the ink generation/undercolor removal processing as a processing conducted to a signal after being subjected to the filtering, using the edge region signal obtained from the signal before being subjected to the filtering as explained above. To be precise, the edge positions do not sometime coincide with each other.
Likewise, an apparatus described in Japanese Patent Application Laid-Open No. H10-173916 performs processings for controlling gamma conversion and halftoning in accordance with one edge amount. The edge amount is generated from a signal before being subjected to the gamma conversion and the filtering that is prior to the gamma conversion. Actually, the line width is increased or decreased by the filtering or the gamma conversion by as small as about one dot and the increase or decrease thereof is not so explicit. However, an apparatus that has a mechanism to cancel outer edges as the apparatus described in Japanese Patent Application Laid-Open No. 2000-206756, requires high positional accuracy, and the increase or decrease of the line width is greatly influential. The significance of the positional accuracy can be easily estimated from a color deviation of each of cyan, magenta, yellow, black (CMYK) plates when a result of image processing is output from any output device such as a printer.
Further, as described in Japanese Patent Application Laid-Open No. H3-16758, there is another apparatus that explicitly controls line width through processing. If a certain adaptive processing based on an edge amount is carried out after line width is processed, by obtaining the edge amount from a signal before being subjected to the line width processing, an edge position may be displaced in the unit of several dots. In this case, adaptive processings in a downstream stage cannot exhibit effects as desired.